Initial freeze–thaw damage detection in concrete using two-dimensional non-contact ultrasonic sensors

•Surface peeling damage to concrete is freeze–thaw damage caused by osmotic pressure.•The dynamic modulus test method was insensitive to the damage at initial freeze–thaw cycle.•The scattering energy of ultrasonic waves was more sensitive to small-scale concrete damage. Freezing and thawing cause concrete deterioration such as micro-cracking and surface scaling. An effective method for initial damage detection in concrete is required to prevent further deterioration of structures. In this study, initial freeze–thaw damage (small-size damage) was detected on a concrete surface using developed two-dimensional non-contact ultrasonic sensors based on a micro-electromechanical system (MEMS). The wavefield of ultrasonic waves passed through damaged concrete surfaces contain propagating and scattered waves, caused by refraction and reflection. To measure the wavefield, multi-channel non-contact ultrasonic sensors was developed including 128 MEMS array. Then, the amplitude of scattered waves was extracted in the frequency-wavenumber domain. The proposed method was experimentally validated through freeze–thaw cycle testing and compared to the conventional dynamic modulus reduction. The results demonstrated that the proposed method was at least 15 times more sensitive than the conventional method toward initial freeze–thaw damage on concrete surfaces.